1. Field of Invention
The present invention relates to anticorrosive paints for metals. More particularly, it relates to paints comprising conjugated polymers as corrosion preventing agents and to methods for the preparation thereof. The present invention also concerns a method of forming a paint coating on the surface of an object and coated metal surfaces comprising an anticorrosive paint composition.
2. Description of Related Art
Corrosion of metal surfaces, such as steel, is a significant problem and in the prior art several ways of different efficiencies have been suggested for solving this problem. The simplest procedure is to passively coat the metal surface with a paint, comprising epoxies, alkyds, polyurethanes, phenolics, and the like. It provides protection if the coating is perfect (defect-free) and does not allow significant diffusion of water, oxygen, electrolytes and the like through it. A common technique is to add additives to paint formulations to provide active protection. Zinc and zinc-containing additives, such as zinc chromates, zinc phosphates, or calcium plumbate, are commonly added to the paints to allow cathodic protection. Detailed description of the prior art of the corrosion-inhibiting pigments can be found e.g. in Paints and surface coating, by R. Lambourne and T. A. Strivens, Woodhead Publishing, Ltd, 1999, pp. 162-165. There has, however, been a push towards reducing or eliminating heavy metal pigments for safety and environmental reasons, and therefore new concepts are called for.
The use of conjugated oligomers and polymers as coatings or additives of coatings to provide anticorrosion effects has been extensively reviewed in the art (W.-K. Lu, S. Basak, and R. L. Elsenbaumer, in Handbook of Conducting Polymers, Marcel Dekker, Inc. 1998, p. 881). It has been suggested to use oligomeric pyrroles or substituted pyrroles as corrosion inhibiting additives (R. M. Hudson and C. J. Warning, Metal Finish. 64 (1966) 63). Also oligomeric aniline has been described to be an inhibiting additive for mild steel such as in hydrochloric acid solution (C. C. Nathan, Corrosion 9 (1953) 199). Oligomeric thiophenes have also been described (Z. Szklarska-Smialowska and M. Kaminski, Corros. Sci. 13 (1973) 1). It can be concluded that conjugated oligomers are shown in the prior art to have corrosion inhibiting properties. However, their toxicity limits their widespread use in practical coatings, as there is a difficulty to bind the oligomers perfectly in the coating to provide mechanical and environmental stability.
Conjugated polymers can in principle be used to overcome this problem. A. G. MacDiarmid suggested use of conducting conjugated polymers to allow anodic protection of steel and other metals (Lecture Series at State University of New York, 1985). D. W. DeBerry (J. Electrochem. Soc. 132 (1985) 1022) found that polyaniline electrochemically deposited on stainless steel provides anodic protection. Since then, there has been considerable effort to develop methods based on conjugated polymers, such as substituted polythiophenes, polypyrroles and polyanilines. There, however, is a major and extensively reported complication due to the rigidity of the conjugated polymer chains: In the general case such materials do not melt and dissolve only in rare or in some cases no solvents due to low conformational entropy (J. I. Kroschwitz, High Performance Polymers and Composites, Wiley, 1991). This central observation can be illustrated using polymers having closely related flexible and rigid modifications: 1) Flexible pyridine containing polymers, such as poly (4-vinyl pyridine) dissolve in a wide variety of solvents, such as alcohols (methanol, ethanol and the like), phenols (m-cresol and other alkyl phenols), halogenated solvents (chloroform, tetrachlorobenzene and the like), amines (N-methyl pyrrolidone, and the like), acids (formic acid, sulphonic acids); 2) Rigid conjugated polymer poly(p-pyridine) dissolves only in few of the solvents listed above, i.e. formic acid (in the unprotonated form), strong acids such as methane sulphonic acid (in the protonated form) and dichloroacetic acid. A commonly used method to identify solvents is based on matching solubility parameters (D. W. Van Krevelen, Properties of polymers, Elsevier, N.Y., 1990, p. 875). The solubility parameters can be evaluated using group contribution methods to render xcex4=22.4 {square root over (MPa)} for poly(4-vinyl pyridine) and a xcex4=21.7 {square root over (MPa)} for poly(p-pyridine), which would suggest that the two polymers should have essentially the same solvents. The fact that only few of the solvents of poly(4-vinyl pyridine) are also solvents for polyp-pyridine) demonstrates that solubility parameters have only a marginal predictive power of identifying solvents for rigid polymers, such as conjugated polymers, like polyaniline. In spite of this well documented inconsistency, there have been efforts to use solubility parameters to identify solvents for polyaniline, as disclosed in U.S. Pat. No. 5,278,213.
Due to the generic poor solubility of the rigid polymers, finding suitable processing routes to produce coatings, in particular anticorrosion coatings, becomes particularly challenging, and preparation of coatings and identifying solvents are intimately connected. It is well known in the prior art that solvents for neutral and doped conjugated polymers are different in the general case. The present application deals with neutral conjugated polymers. Several solvents for undoped (neutral) polyaniline have been experimentally identified and disclosed. U.S. Pat. No. 4,913,867 describes N-methyl pyrrolidone (NMP) as a solvent for neutral polyaniline. Closer studies reveal that such solutions are unstable against gelation upon prolonged storage (K. T. Tzou and R. V. Gregory, Synthetic Metals 69 (1995) 109). Other solvents have also been reported, such as dimethyl sulfoxide, dimethyl formamide, and N,Nxe2x80x2-dimethyl-propylene urea. U.S. Pat. No. 5,278,213 describes a wide variety of solvents and claims solvents with sufficiently high polarity with relative dielectric constant larger than 5, dipole moment greater than 3.5xc2x710xe2x88x9230 Cm, net hydrogen bonding capability greater than ca. 50, and Hildebrand solubility parameter from ca. 17{square root over (MPa)} to ca. 29{square root over (MPa)}. In addition to the problems to classify solvents of rigid polymers by solubility parameters, said patent specification describes solely the use of polyaniline/solvent solutions where all or portion of said solvent is removed from said solution in the end to form solidified neutral polyaniline coatings (col 4 lines 21-24 and col 14 lines 16-24). Further, it should be pointed out that U.S. Pat. No. 5,278,213 does not present or anticipate a case where the solution would be crosslinked instead of its removal.
EP Patent Specification No. 0 581 886 B1 discloses a concept where a neutral polyaniline forms a fine dispersion wit particle size less than 100 nm within a liquid medium where the latter is removed in the end, and where said article is doped for conductivity, unlke in the present invention which deals concepts for neutral non-conducting articles. EP Patent Specification No. 0 623 159 B1 relates to paints based on neutral conjugated polymers dispersed in liquid medium which allows for an anticorrosion effect when the amount of conjugated polymer is from 3 to 20% by weight in the final dried paint, including the binder. As the conjugated polymer is only dispersed as separate particles within the paint without forming more continuous structures (as discussed in the specification of said patent: cf. p. 2, lines 20 and 28), the concentration required to show anticorrosion effect is relatively high. Therefore, there is a need for new ways of obtaining a more uniform distribution of conjugated polymers, not based on dispersions but based on solutions.
U.S. Pat. No. 5,441,772 describes the use of neutral polyaniline solutions to provide coatings which, upon removal of the solvent, render corrosion protection of steel. U.S. Pat. No. 5,441,772 discloses that the composition can include other polymers that are miscible with polyaniline. As neutral polyaniline is immiscible with epoxy resin (see Example 8 below), such as polycondensated bisphenol diglycidyl ethers, the concept is different from the present invention. U.S. Pat. No. 5,658,649 discloses a method to provide two-layer structure where a doped conducting polyaniline is mixed with epoxy resin and cured. Subsequently a second layer, i.e. a topcoat, is added to provide increased mechanical integrity. Unlike our work, U.S. Pat. No. 5,658,649 deals with the conducting form of polyaniline. U.S. Pat. No. 5,645,890 again describe solutions of neutral undoped polyaniline films cast from solvents, such as N-methyl pyrrolidone or dimethyl sulphoxide, upon removal of the solvent. U.S. Pat. No. 5,648,416 describes a wide variety of paint formulations where the neutral undoped polyaniline is dispersed within said paint formulations consisting of one or more binders dispersed within a liquid medium. As solubility is not pursued and polyaniline is in the form of dispersed particles, the required amount of polyaniline is relatively high, as demonstrated in the examples therein. Therefore, new options are called for to provide compositions with smaller amount of polyaniline. U.S. Pat. No. 5,853,621 relates to non-conducting coatings where dispersed polyaniline is used.
The prior art briefly reviewed above describes several concepts to allow anticorrosion coatings based on conducting polymers. Neutral undoped polyaniline is a feasible candidate to inhibit corrosion propagation under nearly neutral conditions. Taking also into account its economics, a wide variety of concepts based on polyaniline has been discussed in the prior art either based on solution casting where a solvent is totally or partly removed at the end to allow solid neutral coating or dispersion where the neutral polyaniline has been dispersed in a wide variety of paint formulations. The first case deals with evaporation of large amounts of volatile potentially harmful organic solvents and the latter case requires relatively large amount of dispersed polyaniline particles to allow anticorrosion effect.
An interesting approach to the manufacture of coatings from non-conducting polyaniline and epoxy resins is discussed by X.-H.Wang et al. In Polyaniline as marine antifouling and corrosion-prevention agent, Synthetic Metals 102 (1999) 1377. According to Wang, a solvent free corrosion prevention coating based on epoxy and neutral polyaniline could be produced by dissolving emeraldine base in tetraethylene pentaamine, which is mentioned as a curing agent of epoxy resin.
Tests made with the amine suggested by Wang, and disclosed in more detail below, have showed, however, that the solution of emeraldine base in tetraethylene pentaamine (TEPA) is not stable. Within a week the solution will turn cloudy, which indicates precipitation and segregation. As a result, the anticorrosive effect of paint compositions incorporating segregated mixtures of tetraethylene pentaamine and emeraldine base will be subtantially poorer, see Example 12 as well as FIGS. 6 and 7.
It is an object of the present invention to provide a novel two-component curable paint composition, comprising a curable binder and a hardener.
It is another object of the invention to provide a method of preparing a two-component paint composition.
It is a third object of the invention to provide a method of forming a paint coating on the surface of an object
It is a fourth object of the invention to provide coated metal substrates with improved corrosion resitivity.
These and other objects, together with the advantages thereof over known methods and products, which shall become apparent from the specification, which follows, are accomplished by the invention as hereinafter described and claimed.
The present invention is based on the finding that polyaniline can be dissolved in a specific group of curing agents comprising an organic amine having primary amine groups to form stable solutions. The amines of the present kind have irregularities in the chemical structure of its hydrocarbon chain. These irregularities comprise either branches or unsaturation in the chain, or both.
Surprisingly, and contrary to the teaching of the prior art cited above, the amines according to the present invention are very effective solvents of polyaniline even if they are primarily less polar and have a rather low solubility parameter.
According to a preferred embodiment, neutral polyaniline is dissolved in one or more basic hardeners of epoxy resins that form stable solutions optionally containing also other additives and the mixture is thereafter cross-linked to form solid surface coatings.
The method of forming a paint coating on the surface of an object typically includes the steps of
i) dissolving neutral polyaniline in an epoxy hardener, optionally containing also other additives;
ii) mixing epoxy resin, optionally containing also other additives, in the said polyaniline/hardener mixture,
iii) optionally adding further additives;
iv) applying the mixture on the surface of an object; and
iii) curing the coating
More specifically, the present invention is mainly characterized by what is stated in the characterizing part of claim 1.
The method according to the present invention of preparing paint compositions is characterized by what is stated in the characterizing part of claim 25.
The method of forming a paint coating on the surface of an object is characterized by what is stated in the characterizing part of claim 32, and the coated metal surface is characterized by what is stated in the characterizing part of claim 37.
Considerable advantages are obtained. Thus, the present invention represents a straightforward and economically attractive procedure for producing coatings and coated articles based on solutions of neutral undoped polyaniline. Instead of solvent removal the invention provides for cross-linking of the polyaniline using polymeric or oligomeric resins to produce neutral polyaniline coatings that have anticorrosion effect at lower concentration of said polyaniline than allowed by dispersion techniques.
The solutions produced by the invention are stable over extended periods of time, which facilitates storage, and transportation of the paint compositions. The anti-corrosive effect of the paint compositions is good even at very low concentrations of the polyaniline. Surprisingly it has been observed that tie required amount of polyaniline is low, typically about 1% by weight, to allow for a substantial anticorrosion effect.